The paper focuses on measuring and assessing the relation of adaptive maintenance process and quality of open source software (OSS). A framework for assessing adaptive maintenance process is proposed and applied. The framework consists of six subprocesses. Five OSSs with considerable number of releases have been studied empirically. Their main evolutionary and quality characteristics have been measured. The main results of the study are the following:. 1) Software maintainability is affected mostly by the activities of the 'Analysis' maintenance sub-process. 2) Software testability is affected by the activities of all maintenance sub-processes. 3) Software reliability is affected mostly by the activities of the 'Design' and 'Delivery' maintenance subprocesses. 4) Software complexity is affected mostly by the activities of the 'Problem identification', 'Design', 'Implementation' and 'Test' sub-processes. 5) Software flexibility is affected mostly by the activities of the 'Delivery' sub-process.